The objectives of the proposed research are to obtain a detailed understanding of the physiological functioning and metabolic regulation of a highly branched biosynthetic pathway and to learn the physiochemical mechanisms of catalysis and regulation of this system. The enzyme system chosen is the phosphoribosylpyrphosphate synthetase of Salmonella typhimurium, which catalyzes an unusual pyrophosphate group transfer reaction and which is required for synthesis of a large group of nucleic acid precursors, two amino acids, and two coenzymes. Regulation of synthesis and activity of the enzyme will be investigated by study of appropriate mutant strains and by kinetic and physical studies of the highly purified enzyme. Evaluation of the physiological functioning of PRPP synthetase in living cells will be attempted by determination of PRPP pools under various conditions and selection of a temperature-sensitive mutant for the enzyme. The mechanisms of catalysis will be examined by a variety of kinetic and physiochemical techniques. In the broadest sense this research is directed toward a better understanding of the regulation of cellular metabolism, the mechanism of biochemically important transfer reactions, and biochemical mechanisms of metabolic diseases, such as hyperuriemia, which are characterized by derangements in nucleotide metabolism and which involve PRPP synthetase in some cases.